Facebook Music is launching tomorrow. This, everyone knows. But not everyone knows exactly how it will work, since Facebook been tweaking things right up until launch. But tonight one Facebook employee accidentally revealed one killer feature of the service.

Ji Lee, the creative director that Facebook stole away from Google in April, sent out the following tweet earlier:

The “Listen with your friend” feature in ticker is blowing my mind. Listen to what your friends are listening. LIVE.

Within minutes, Lee deleted the tweet, but not before Mitchell Holder grabbed a screenshot. Yes, Lee is describing a key feature of Facebook Music launching tomorrow. Not only will all music you’re listening to appear in the just-launched right-side ticker, there will be a link to “Listen with your friend”, that when clicked, will allow you to listen along to the same song at the same time (thanks to the magic of scrobbling and track matching).

Undoubtedly, this feature is already live on Lee’s Facebook account, and he probably didn’t realize he wasn’t supposed to talk about it yet. Earlier today, Lee used his Facebook account to leave the status:

Ji Lee is listening to Black & Blue by Mike Snow and feeling happy it’s another beautiful sunny day in California.

Yep. Also Facebook Music.

One other thing we can confirm: the unified music “remote control” on Facebook has been removed, as Peter Kafka first reported yesterday. This means that in order to control the music (play/pause/stop/next track/etc) you’ll have to switch windows to the music service itself.